1. Technical Field of the Invention
The present invention relates to a semiconductor laser device, being a semiconductor laser module having a semiconductor laser element (hereinafter, referred to as “LD”), and a photodiode (hereinafter, referred to as “PD”) for monitoring the light power of the LD, and relates in particular to an arrangement and a structure of a PD for a monitor (hereinafter, referred to as “monitor PD”).
2. Description of the Background Art
Semiconductor laser devices mainly used in optical communication, for example, as disclosed in the following Patent Documents 1, 2, and 3, are often provided with a mechanism to monitor the optical power of a LD in order to stabilize the optical power of the light output of the LD. In this case, the forward optical output of the LD is generally used for an optical signal, while the rearward optical output is received by a monitor PD to control the driving electric current of the LD so that the forward optical power becomes constant.
[Patent Document 1] Japanese Unexamined Patent Publication No. Hei 6-289258
[Patent Document 2] Japanese Unexamined Patent Publication No. 2003-303975
[Patent Document 3] Japanese Unexamined Patent Publication No. Hei 9-270566
In general, as an installation method of the monitor PD, as described in Patent Document 1, the monitor PD is mounted on a subcarrier for a PD. The LD is mounted on a subcarrier for a LD. On a laser carrier, the subcarrier for a PD is arranged behind the subcarrier for a LD. These subcarriers are fixed on the laser carrier. The subcarrier is a supporting base and is sometimes called a submount, a base, or a header.
However, in the above installation method, the number of parts increases, leading to a problem of an increase in the number of assembly processes. An even larger problem is that the size of the subcarrier for the PD is significantly greater than that of the LD.
Recently, in a semiconductor laser device of a shape in which the direction of the optical output and the direction of a pin for extracting electric signals (leading terminal) oppose each other, a signal line for the LD, which drives the LD, needs to be arranged so as to avoid the subcarrier for a monitor PD. In the case where the frequency for driving the LD is, for example, 2.5 GHz or less, avoiding this signal line for the LD is not a significant problem. However, in the case where the frequency for driving the LD is 10 GHz or more, the wiring pattern of the signal line for the LD exerts a significant influence on the transmission of electric signals, and if the signal line is arranged so as to avoid a large-sized subcarrier for the monitor, the frequency characteristics of the semiconductor laser device are significantly impaired.
As a means to solve this problem, as described in Patent Documents 2 and 3, there is a method of directly installing the monitor PD rearward of the LD without using the subcarrier for the monitor PD.
In Patent Document 2, a LD is fixed on a silicon (Si) substrate, and a PD is fixed within a trench (concave section) formed on the Si substrate to the rear of this LD. A light receiving section of this PD is arranged in a position lower than the LD.
In Patent Document 3, a PD is formed on a surface of a semiconductor substrate, and a LD is fixed on the semiconductor substrate so as to be adjacent to and higher than this PD. Then, a prism for light-focusing is provided above the light receiving section of the PD, and the optical output from the rearward of the LD is incident on the light receiving section of the PD via the prism.
However, in any one of the methods disclosed in Patent Documents 2 and 3, processing needs to be performed on the substrate on which the LD is mounted. Moreover, in order to obtain a stable monitor PD electric current, high precision installation of the monitor PD is required. As a result, there has been a problem of an increase in the number of manufacturing processes and a resultant high manufacturing cost.